CVE-2026-55199— libssh2 - Pre-Authentication DoS via SSH_MSG_EXT_INFO Handler
Possible ATT&CK Techniques 1AI
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I. Basic Information for CVE-2026-55199
Vulnerability Information
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Vulnerability Title
libssh2 - Pre-Authentication DoS via SSH_MSG_EXT_INFO Handler
Source: NVD (National Vulnerability Database)
Vulnerability Description
libssh2 through 1.11.1, fixed in commit 1762685, contains a pre-authentication denial of service vulnerability in the SSH_MSG_EXT_INFO handler in src/packet.c that allows a malicious SSH server to cause a client CPU exhaustion loop by sending a crafted extension count value. A malicious server can set nr_extensions to 0xFFFFFFFF during key exchange, causing the client to spin in a tight CPU loop for over 60 seconds because return values from _libssh2_get_string() are unchecked and the session timeout does not apply to CPU-bound loops.
Source: NVD (National Vulnerability Database)
CVSS Information
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
Source: NVD (National Vulnerability Database)
Vulnerability Type
不可达退出条件的循环(无限循环)
Source: NVD (National Vulnerability Database)
Vulnerability Title
libssh2 资源管理错误漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
libssh2是libssh2团队开源的一款实现SSH2协议的客户端C库。 libssh2 1.11.1及之前版本存在资源管理错误漏洞，该漏洞源于在src/packet.c的SSH_MSG_EXT_INFO处理程序中存在预身份验证拒绝服务漏洞，允许恶意SSH服务器通过发送特制的扩展计数值导致客户端CPU耗尽循环。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)
Affected Products
II. Public POCs for CVE-2026-55199
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Patches & Fixes for CVE-2026-55199 (2)
Vendor Advisories for CVE-2026-55199 (1)
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V. Comments for CVE-2026-55199
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